Effects of αCGRP and βCGRP on the cranial circulation
We studied whether rat-αCGRP and βCGRP elicited the same effect on the peripheral and cerebral hemodynamics and investigated the ability of BIBN4096BS to prevent their vasodilatatory properties. A dose of 0.3 μg kg−1 of CGRP was chosen for the inhibition studies. The choice was based on the dose–effect relationship data obtained in this study (Figure 1) and existing knowledge of a submaximal and reproducible effect on the MMA of this dose (Williamson et al., 1997b; Honey et al., 2002). In the present study, αCGRP and βCGRP were equally potent and dose-dependently dilated both the MMA and the PA. This in agreement with previous findings showing that there are no major differences in the response of cranial arteries (human or animal) to the different subtypes of CGRP (Jansen-Olesen et al., 2003). The somewhat less-pronounced relaxation of the PAs by CGRP may be explained by the concomitant hypotension, since we and others have documented a correlation between PA diameter and hemorrhagic-induced hypotension (Kontos et al., 1978; Petersen et al., 2004a).
Studies investigating the effect of systemic CGRP on CBF have revealed conflicting results (Beattie et al., 1993; Baskaya et al., 1995). In most species including the rat, the peptide may increase CBF in response to i.v. administration (Suzuki et al., 1989; Baskaya et al., 1995). Our data supported this and showed that infusion of αCGRP or βCGRP, the former being more potent, dose-dependently increased LCBFFlux.
Mechanism and site of action of BIBN4096BS
BIBN4096BS is a relatively large hydrophilic compound and is therefore unlikely to pass the blood–brain barrier (BBB) in acute experiments. However, no direct data exist to support this. In contrast to cerebral vessels, the meningeal arteries have no BBB and BIBN4096BS is likely to diffuse freely into the wall of the MMA (Knudsen et al., 1988; Faraci et al., 1989). BIBN4096BS has been shown to have a high affinity for the CGRP1 receptor in vitro (Doods et al., 2000; Edvinsson et al., 2001b; 2002; Schindler & Doods, 2002). Since receptors with a CGRP1 pharmacological profile are primarily situated on the smooth muscle cells of cerebral arteries (Edvinsson et al., 2002; Oliver et al., 2002; Petersen et al., 2004b), it is not clear whether BIBN4096BS has any effect on cerebral arteries in vivo. However, an endothelial CGRP receptor may exist, since CRLR, RAMP1, RAMP2 and receptor component protein are expressed in microvascular endothelial cells and in human cerebral arteries, in addition RAMP3 (Edvinsson et al., 2002; Moreno et al., 2002; Oliver et al., 2002; Jansen-Olesen et al., 2003), but there are no functional data to support this.
We found that the MMA dilatation in response to systemically infused CGRP was blocked and the vasodilatation induced by TES (endogenously released CGRP) was markedly inhibited. Activation of the trigeminal ganglion by electrical stimulation releases primarily CGRP, but also, to a minor extent, substance P and PACAP and other peptides to the abluminal side of the vessels (Goadsby et al., 1988; Zagami et al., 1990; Williamson et al., 1997b). These co-released peptides may account for the remaining 20% of the dilatation not inhibited by BIBN4096BS. A higher local CGRP concentration and unspecific effects of TES are alternative explanations for the lack of complete inhibition. The inhibitory effect of BIBN4096BS was equally potent towards the αCGRP and βCGRP response, this in disagreement with previous in vivo findings (Wu et al., 2000), however in agreement with the larger part of existing in vitro data (Jansen-Olesen et al., 2003).
The PA investigated did not exceed a size of 100 μm and can be defined as second-order arterioles (Harper et al., 1984). The αCGRP-induced PA vasodilatation was only nonsignificantly reduced after the highest doses of BIBN4096BS. The antagonist did not inhibit βCGRP and the neurogenically evoked PA dilatation. Furthermore, BIBN4096BS did not significantly inhibit the increase in LCBFFlux seen after intravenously administration of CGRP.
The difference in inhibitory effect of BIBN4096BS on MMA and PA raises an interesting subject for further discussion. As outlined previously, the two types of arteries express a different vessel wall structure. The PA directly investigated and the smaller cerebral vessels indirectly measured by LDF are believed to posses a BBB. In theory, this would mean that BIBN4096BS could not pass the BBB and hence could not bind to the cerebrovascular CGRP receptors (Petersen et al., 2004b). The influence of the BBB was supported by the inability of BIBN4096BS to block the effect of TES on PA vasodilatation. With increasing doses of BIBN4096BS, the αCGRP-induced PA dilatation decreased. The reduction in PA dilatation is most likely to be secondary to the prevention of the induced hypotension. It is unlikely to be a direct effect of the antagonist. Since βCGRP induced a less-pronounced degree of hypotension (approximately 10 mmHg), a similar reduction in PA dilatation was not seen for βCGRP. The origin of the minor dilatation seen in PA after infusion of the lower doses of CGRP with only limited effect on the blood pressure is not readily explained. The application of TES induced a dilatation of PA without any effect on the MABP.
It has been proposed (Markowitz et al., 1988; Williamson et al., 1997b) that the dilatation of MMA induced by electrical stimulation mainly is mediated by CGRP. This was further supported by the use of BIBN4096BS in this study. Electrical stimulation of the superior sagital sinus in the cat (Goadsby et al., 1991) and the dura mater in rats (Kurosawa et al., 1995) leads to an increase in cerebral blood flow, an increase found to be mediated by CGRP. Based on existing data, it is known that PAs are innervated by CGRP containing C-fibres originating in the trigeminal ganglion (Edvinsson et al., 2001a) and furthermore that PA dilatation is mediated through CGRP receptors (McCulloch et al., 1986; Wei et al., 1992; Hong et al., 1994). It is therefore most likely that the TES-induced PA dilatation in this study was caused by CGRP release, however no direct evidence is obtained due to the ineffectiveness of BIBN4096BS.
The results from these experiments contribute to the understanding of the site of action for BIBN4096BS. It seems that BIBN4096BS does not pass the BBB in the rat, but is very effective in preventing CGRP-induced vasodilatation in vessels without a BBB.
The present study strongly suggest that the clinically effective migraine drug BIBN4096BS (Olesen et al., 2004) does not cross the BBB. With the caution of species differences in BBB function or the possible occurrence of transient BBB changes during the migraine attack, this indicates that dural arteries may play an important role in migraine pathogenesis.